System of light modulation



947. EC. THOMSON 2,432,104

SYSTEM OF LIGHT MODULATION Filed March 10, 1945 INVENTOR.

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Patented Dec. 9, 1947 SYSTEM OF LIGHT MODULATION Elihu Craig Thomson, Boston, Mass, assignor to Photoswitch, Inc.. Cambridge, Mass, a corporation of Massachusetts Application March 10, 1945, Serial No. 582,112

3 Claims. 1

The present invention relates to systems requiring modulation of the light emission from electric light sources, and more particularly to an electric circuit incorporating arrangements for periodically interrupting the light of an electric lamp that affects a light-responsive device.

It is the main object of the invention to provide an electric circuit which in an extremely simple and reliable manner modulates the output of an electric lamp which controls light-sensitive electric apparatus.

It is often desirable to make use of a light source whose emission is subject to periodical fluctuations which are large enough to aifect light-sensitive devices such as phototubes.

One of the reasons making such a light source desirable is that of using it for the control of a photoelectric device which is responsive only to light of a given frequency, thus eliminating interference from other light sources in the vicinity of the phototube. Another reason to use fluctuating light sources is the well-known fact that proper amplification of the output of phototubes can best be accomplished by means of alternating current amplification circuits. Various devices have been proposed to obtain a pulsating output of phototubes. One of these is the so-called mechanical light chopper which usually consists of a slotted disk rotated in front of the phototube. Other ways of accomplishing similar results involve the use of auxiliary photocells and light sources in an oscillating circuit, the use of Kerr cells and light-polarizing apparatus, or the use of self-oscillating phototube circuits. All these arrangements have disadvantages such as movable mechanical parts, electron discharge devices added to already rather complex electronic amplification circuits, inherently uncertain operation, and introduction of undesirable higher harmonics.

The arrangement according to the present invention avoids such difiiculties by incorporating a purely electric arrangement in the supply circuit of the light source which excites the photosensitive apparatus.

Although the above-indicated use of the arrangement according to the invention in cooperation with photoelectric circuits is especially beneficial, it is understood that the effect obtained may be of use in other environments, wherever a fluctuating light source is desirable.

These and other objects and aspects will be more fully apparent from the following descrip tion of a practical embodiment illustrating the general character of the invention. The description refers to a drawing in which Fig. 1 is a circuit diagram of a practical embodiment of the invention; and

Fig. 2 is a diagram illustrating the operation of the device according to Fig. 1.

Fig. 1 shows at L a transformer whose primary Lp is supplied from an alternating current source indicated by terminals A, B. This source will in most instances be a standard (SO-cycle alternating current line but it is understood that alternating or fluctuating current sources of different frequencies can be employed and that frequency modifying devices such as frequency reducing or multiplying arrangements may be inserted between the available alternating current source and the circuit according to the invention.

The secondary Ls of transformer L is connected to a rectifying circuit which, in the present example that employs a standard incandescent lamp furnishing light sufficient for operating a standard phototube, may consist of two standard, type 3576, rectifier tubes TI and T2 connected in parallel, and having heaters h, cathodes k and anodes a. The heaters h may be supplied from a tap t of secondary Ls through a voltage-reducing and current-limiting resistor Ri. The cathodes 7c are connected at d to terminal b of secondary Ls and to the heater circuit. The anodes a lead through load-equalizing resistors R2, R3, R6, R5 to one terminal of light source ID in the present instance a standard volt 50 watt incandescent lamp, whose other terminal is connected to terminal a of secondary Ls.

The light source is preferably an incandescent lamp but, in combination with suitably selected photoelectric circuits, any radiation source may be employed which is fundamentally slow heating to such a degree that the radiation loss between the peaks of the conventional alternating current source would provide a light modulation insuflicient for purposes of efllciently operating the device which is to be controlled by the .radiation source.

The light source I 0 is arranged to illuminate in suitable, conventional manner a phototube P which forms part of an amplification and control network indicated at N. In accordance with the purpose of the invention, this network will be one which is especially suited for alternating current operation of a sensitivity adapted to the range of variation of radiation as provided in accordance with the present invention, and it may include arrangements for discriminating between different frequencies. I

The arrangement according to Fig. 1 operates a follows.

The alternating current furnished by secondary Ls has the wave shape indicated in Fig. 2 by dotted lines below, and in full lines above the zero axis. Since the lamp filament is heated by current flowing in either direction, the light source would have a modulation frequency of 120 cycles, if it were connected directly to the transformer L. With'alight soiirce 'of'th'eabova indicated cliaractristics, this modulation is not sufiiciently distinct to be detected in ordinary pho toelectric circuits, because the filament does not suificiently cool to affect the light radiation anpreciably in the short time during which no current is flowing.

The rectifying circuit according to "theinven tion eliminates in known manner the half cycles shown in dotted lines in Fig. '2 and provides current of the wave shape indicated with a heavy line. The current intensity is zero during periods essentially corresponding to the eliminated half cycle and these periods are df-sufiicie'nt nuration to permit the filamentt'o can between the current peaks, thus reducing the light radiation from the filament appreciably between these peaks, The modulation thus produced is sufficient fordetection by the conventional phot'otub'e circuit of the aboveindicated nature,;ihte'rided for'alternating'current bp'eration and, ifdesired, for frequency discrimination. It is understood that'the rectifier-system'can be connected in various ways for egiarnple for shunting one half '"wave serie's around the light source. V h

It will be evident that "apparatus-of this type can be used not only for detecting persons or articles passing betweenligh't source in and photo tube P, but that any detecting, light absorbing or otherwise modifying device can bein'se'rted between lamp and light sensitive element. v

it should be understood that thep'resent dis closure is for the-purpose of illustration only-and that this invention includes all modifications and equivalents which ran within "the scope of the appended claims.

1. Apparatus for providing are'g'ularly m d; iated 'light flux, comprising: an alternatin 'c'urrent-supply, a lightsource adapted to maintain a "slightly varyin lightjiitput "while sjlipplled with current impulses at the intervals of thelh'alf wave periods of said supplybut to'furnish'afiuv tuating light output while supplied 'with current impulses at intervals of'alte'rnat'e half wave periodsof 'said supply,a transrormerhaymg'a prrmaryoonnect'ed to said supply "and a secondary with two terminals, electron discharge in'ean's having cathode means and anode means and adapted to pass current in one direction, and connections including saidlight source and forming a circuit from oneof said terminals through said cathode and anode means to thefotli er te'rminaL the unidirectional conductivity (if "said discharge means causing current impulses to be supplied to said light source at alternate ha1f wave periods of said su' my and hence causin supply, 'a light'output fluctuating periodically at the supply frequency; a half wave rectifier; and a transformer having a primary connected to s'aidsupply "and a secondary; said secondary,

's'aidlialfwaiiie'ietitifier and said lamp being con- -'neted-in =a"'series*circuit; said half wave rectifier passing only alternate half wave impulses frtir'n sale 'supply to said lamp and thereby causing said lampto furnish a light output fluctuating p'' odially at the supply -fr'e'quency.

1ect1-ica pparatus comprising in combinatidn, an'op'erativeeircuitresponsive substantially only "to signals of "a predetermined frequency aha including 'a light-sensitive actuating member; and a controlling circuit including an alternating current supply of substantially said predetermined frequency, an incandescent lain'p having =arsistajnee "filanint arrangeoror ili'uinitiation of said light Sensitive actuatin 'mein her, said filamentbeing adapted-to maintain a slightly varying light outputwhile sup lied with cfirrent imbulsesat substantially double said predetermined frequency, but adapted to furnish, while supplied with "current impulses at'said predterrnined fredi'lhl', a light output fluctuating periodically at said predetermined frequency, a half wave rectifier, and a transformer having a primary connected'tosaidsupplyand a secondary; saidsecondary half wave rectifier'and said lain'p being connected in 'a series circuit; whereby said half-wave 'rectifier'pa'sses only alternate half wavecurr'ent impulses from said supply'to;said

, lamp "and hence causing saidlanip to furnish light fluctuating "periodically at said predetermined frequency'to said light-sensitive actuating member.

CRAIGTHOM'SON.

CITED 7 The-following references areof record in'the -file of this patent: 

